Waveguide filter



Oct. 22, 1957 R. w. KLOPFENSTEIN 2,810,890

WAVEGUIDE FILTER Filed NOV. 23, 1954 INPUT IN VEN TOR. RALPH WALoPf/Vs T51/V BY ,f5

/Q 55m A QTTOR'A/EY United States Patent WAvnGUmE rumen Ralph W. Klopfenstein, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application November 23, 1954, Serial No. 470,661

Claims. (Cl. S33- 73) The present invention relates to electromagnetic wave filters and in particular to improved waveguide lilters.

lt is an object of the present invention to provide an improved, high-power filter suitable for use in the ultra high frequency and hiher regions of the radio frequency spectrum.

lt is another object of the present invention to provide an improved, hollow pipe waveguide filter which passes wave energy at a fundamental frequency substantially without loss and which substantially fully attenuates energy at certain higher frequencies.

A typical embodiment of the present invention includes a main waveguide dimensioned to support the propagation of a given mode of plane polarized wave energy at a fundamental frequency and substantially higher frequencies and a second waveguide dimensioned to support only the propagation of wave energy at the substantially higher frequencies. The second waveguide is joined to the first by means of a tapered transition section, whereby wave energy at the fundamental frequency is reflected back from the tapered section and wave energy at frequencies substantially higher than the fundamental frequency passes through the transition section and into the second waveguide. The second waveguide may be terminated in a dissipating load for dissipating the higher frequency energy or, if desired, may lead to an output device.

Located between the input end of the main waveguide and the transition section is a circular polarizer. This converts the input plane polarized energy to circul-arly polarized wave energy, and recouverts the retlected, circularly polarized wave energy to plane polarized energy. The reflected plane polarized wave energy is in space quadrature with the incident plane polarized energy whereby it may be extracted from the main waveguide by an output device which is suitably oriented.

ln one form of the invention the input means to the main waveguide includes a rectangular waveguide oriented with its broad dimension parallel to the longitudinal axis of the main waveguide. The output means is also a rectangular waveguide oriented with its broad wall parallel to the longitudinal axis of the main guide but is in space quadrature with the input waveguide.

A preferred embodiment of the invention includes a septum in the main guide oriented to prevent harmonic energy in certain modes higher than the dominant mode from being transmitted toward the remote end of the guide. The septum is located between the input means and the output means `and is spaced an odd number of guide quarter wavelengths from the output means. The septum therefore also prevents the reflected, fundamental plane polarized energy from travelling past the output means as, with respect to this energy, the septum is a short circuit and appears at the output port looking toward the septum as an open circuit.

The invention will be described in greater detail by reference to the following description taken in connection with the accompanying drawing in which:

Fig. 1 is a longitudinal, cross-sectional view of a typical embodiment of the present invention;

Fig, 2 is a plan view of Fig. l;

Fig. 3 is a cross-section along line 3 3 of Fig. 1;

Fig. 4 is a cross-sectional view of a second embodiment of the invention shown in Fig. l; and

Fig. 5 is a cross-section line 5 5 of Fig. 4.

ln the figures, similar reference numerals designate similar elements.

Referring now to Figs. l, 2 and 3 rectangular waveguide 16 is electrically coupled to circular waveguide 12 with its broad wall parallel to the axis of the circular waveguide. The circular waveguide is dimensioned to support the propagation of wave energy in the TE11 mode. The input wave may include a fundamental frequenCy fo and harmonics of the fundamental frequency 2te, 3ft, etc. With the orientation of rectangular and circular waveguides shown, the input rect-'angular waveguide l@ excites the circular waveguide in the TEn mode and the latter propagates plane polarized horizontal wave energy to the right. The closed end 14 of the circular waveguide is spaced approximately an odd number of guide quarter wavelengths, at the fundamental input frequenc from the input waveguide 1li. The fundamental energy entering the circular waveguide sees an open circuit to the left and therefore is propagated to the right.

The output waveguide 16 is in space quadrature with input waveguide 10 and, as the waves propagated past the output waveguide are horizontally polarized they are not coupled to this waveguide. The output waveguide aperture, in other words, is in a neutral plane with respect to the plane polarized wave travelling t0 the right. The output waveguide is spaced an integral number m of guide half wavelengths from the input waveguide 10.

Ridges 1S, 18' comprise a circular polarizing section and they convert the horizontally polarized TEn wave to a circularly polarized TE11 wave. These are preferably tapered at the ends 19 thereof and are located at an angle of 45 with respect to the longitudinal axis 21 of the output waveguide 18. Although shown as metal ridges, it is to be understood that other polarizers known to those skilled in the `art such as metal or dielectric rods or dielectric slabs may be employed for the same purpose.

Circular waveguide 20 which is of substantially smaller cross-sectional conguration than the main waveguide 12 is located at the remote end of the latter. Guide 20 is joined to the main waveguide by 'a tapered, broad-band transition section 22. The dimensions of waveguide section Ztl are below that which will support the propagation of wave energy at the fundamental frequency fo but are sutliciently large to support the propagation of harmonic wave energy at frequencies 210, 3ft, etc. Thus, substantially all harmonic energy passes into waveguide 2l) and is dissipated by the high frequency dissipating load 24- which may consist of Bakelite, ferrous metal or other types of lossy material.

Substantially all wave energy at the fundamental frequency which is incident on transition section 22 is reected back toward the input end of the waveguide. The circular polarizing section 18, 18 recouverts the reected, circularly polarized wave energy into plane polarized energy but changes the plane of polarization by so that the reflected, plane polarized energy is vertically polarized. Output rectangular waveguide 16 is mounted to the circular waveguide with the broad wall thereof parallel to the longitudinal axis of the waveguide. Accordingly, the vertically polarized, reflected wave energy now excites the output waveguide, and the latter passes substantially all energy at the fundamental frequency to the output load.

The above-described device is usable at extremely high U. H. F. band is believed to be on the conservative side. The harmonic rejection of the filter exceeds 43 decibels when an -input voltage standing'wave ratio (VSWR) of 0.98or better is maintained at thecircu'lar polarizing section 18, 18' for harmonic frequencies.

Figures 4 and 5 illustrate a'second embodiment of invention which is similar to the embodiments of Figures 1-3 except for member 30. The latter is a metal plate and located in the center of the waveguide with its surfaces perpendicular to the axis of output waveguide 16. The edge 32 of the plate closest to the output waveguide is spaced therefrom an odd number of guide quarter wavelengths.

Metal plate 30 serves two important functions. First, it prevents higher order modes such as TMol and TMoz modes of harmonic energy from being propagated down the main waveguide 12 toward the output waveguide. Second, it `substantially prevents reccted, plane polarized energy from being propagated past the output waveguide 16 toward the input end of the waveguide. The plate is so'oriented that it acts asa short circuit to the reflected plane polarized wave energy. Since the output port is spaced an odd number of guideV quarter wavelengths from the edge 32 of the plate, looking from the output port the plate appears as an open circuit. Thus, the use of the plate allows input and output means Vand 16 separately to be matched. In the embodiments of Figures l-3, on the other hand, both input and output waveguides must independently be matched with respect to the short circuiting end 14 of the waveguide.

'Although the preferred forms of the invention are described in terms of a filter which passesa fundamental frequency and which rejects harrnonically related frequencies, it is to be understood that the inventive concept is equally applicable to other types of filters. Thus, with proper design of broad band tapered transition sec- Vtion 22 and narrow circular waveguide 20 a lter may be designed to pass Va fundamental frequency and reject all'frequenciesV 1% or less times greater than the fundamental frequency. Moreover, looking at the invention from a slightly different aspect, if a utilization device is substituted for the dissipating load.V 24, the filter is usable to provide harmonic frequencies at one output thereof and a fundamental frequency at another output thereof. Finally, in another aspect of the invention, a dissipating load may be located in the output rectangular waveguide 16 and an output. load such as an antenna or other high frequency utilization circuit substitutedfor dissipating load 24 in waveguide 20. In this last case, the filter is a high pass filter which substantially fully attenuates all energyat the`fundamental frequency and which passes allharmonically related frequencies or, in the broader aspect, all substantially higher frequencies.

In the foregoing description the main waveguide is described as a circular waveguide. This is meant merely tofbe illustrative. It will be understood by those skilled in the art that other types'of waveguides may be employed instead, such as, for example, square waveguides which support the propagation of plane Land circularly polarized wave energy. VIt is alsoV to be understood that although the input and output means are shown as being rectangular waveguides other equivalent devices could be substituted. For example, the input to the main waveguide may comprise a rectangular waveguide joined to the'input end 14 of the waveguide by means of a broadband tapered transition section. .Moreoven at Ylower power levels, coaxial line'inputs and outputs may be em- YployedV as will be evident to those Vskilled in the art.

"What is claimed is;

l. A--waveguide filter comprising, in combination, a main, hollow pipe waveguide; input means electrically coupledto said waveguidefor exciting thepropagation therein of'plane polarized Wave'energy at a fundamental frequency and at frequencies substantially higher than said fundamental frequency; a second waveguide dimensioned toY support the propagation of wave energy at said higher frequencies but not at said fundamental frequency; a transition section located between said main waveguide and said second waveguide and joining the ytwo together, whereby wave energy at said higher frequencies pass through said transition section into said second waveguide and wave energy at said fundamental frequency is reflected back from said transition section; a circular polarizer located in said waveguide between said input means and said transition section for con-V verting the plane polarized energy travelling from lsaid input means to said transition section to circularly polarized wave energy and for reconverting the circularly polarized wave energy reflected back Yfrom said transition section toward said input means to plane polarized wave energy in space quadrature with the plane polarized wave energy travelling from said input means toward said circular polarizer; and output means electrically coupled to said main waveguide between said input means and said circular polarizer for receiving said reflected, plane polarized wave energy.

2. A waveguide lter as set forth in claim l, said mainY hollow pipe waveguide comprising a circular waveguide and said second waveguide comprising a second circular waveguide, this one of small cross sectional configuration than said main hollow pipe waveguide.

3. A waveguide filter as set forth in claim 1, and further including a dissipating load locatedV in said second waveguide. Y

4. A waveguide lteras set forth in claim l, wherein said main hollow pipe waveguide and said second waveguide are circular waveguides, said input means comprises a rectangular waveguide positioned with the broad wall thereof aligned with the longitudinal axis of said circular waveguide and said output means comprises a rectangular waveguide in space quadrature to said input waveguide `with-,the broad wall thereof parallel to the longitudinal axis of said waveguide.

5. ;A waveguide filter as set forth in claim 4, said circular polarizer comprising a pair of metallic ns mounted to the inner walls of said main waveguide with the long dimensions of the fins parallel to the longitudinal axis of said main waveguide, said fins being spaced apart and lying in a plane at 45 to the'broad wall of said input waveguide. Y

6. A waveguide filter as set forth in claim 5, and fur- Vther including a tapered ,dissipatingload located in said second waveguide with the narrow end of said tapered loadV extending toward said transition section.

7, A waveguide filter as set forth in claim 6, said Vtransition section comprising a conically shaped member joined to said main'waveguide at the larger end thereof and to said second waveguide at the smallerend thereof.

8. A waveguide filter as set forth in claim 7, the end of said main waveguide remote from said Vsecond waveguide comprising a conductive plate spaced approximately au odd number of quarter guide wavelengths from said input means at said fundamental frequency.

9. A waveguide lter as set forth in claim l, and further including means located between said input' means Vand said output means for preventing the propagation along said main waveguide toward said transition section of harmonic energy in modes higher than a given mode. Y

l0. A waveguide Llter as set forth in claim l, and further including a metal plate joined at Vopposite ends thereof to the inner surfaces of said main waveguide, said plate being located between said input means and said 'outputrmeans and being spaced an odd number of guide quarter wavelengths, at said fundamental frequency, from said output means, and said-plate being oriented to present a short circuit to said reflected, plane polarized wave energy, wherebyv looking lfrom said output means toward said plate Vsaid reflected plane polarized energy Refeaences Cited in the le of this patent UNTED STATES PATENTS Feldman Jan. 11, 1949 6 Martin july 11, 195() Purcell et a1 Aug. 19, 1952 Lamont et al Sept. 8, 1953 Gibson May 24, 1955 Farr July 12, 1955 

